Examples of recent processes for producing a silver halide emulsion and apparatuses therefor include the following:
(1) An apparatus for the continuous production of an AgX emulsion in which an aqueous silver salt solution and an aqueous X.sup.- salt solution are continuously fed by a double jet method in the presence of a dispersion medium and the AgX emulsion obtained is continuously withdrawn from the reaction vessel during the formation of AgX grains, and an apparatus for the continuous production of an AgX emulsion in which an AgX emulsion continuously removed from a reaction vessel in cascade type reaction apparatuses connected with each other is used as a feed material for the subsequent reaction vessel.
Such conventional processes and apparatuses are described in V. L. Zelikmen and S. M. Levi, Making and Coating Photographic Emulsions, p. 228 (Focal Press, London) (1964); U.S. Pat. Nos. 3,773,516 and 4,046,576; and K. Ariga, Journal of the Society of Photographic Science and Technology of Japan, Vol. 30, 99 (1967).
(2) An apparatus for continuous production in which an AgX emulsion is continuously passed through a tube or a pipe and a number of inlet ports for an aqueous silver salt solution and an aqueous X.sup.- salt solution are provided midway between the top and bottom of the tube or pipe, as described in U.S. Pat. Nos. 3,655,166 and 3,827,888 and West German Patent (OLS) No. 2,755,166.
In the first process and apparatus described above, however, the withdrawn AgX grains differ in residence time in the reaction vessel, since the AgX emulsion is continuously withdrawn during the continuous nucleation. Thus the AgX emulsion obtained has a wide grain size distribution. Furthermore, the size distribution varies with the lapse of time. A constant size distribution can be achieved by shortening the residence time. In this case, however, the average grain size is undesirably reduced, as disclosed for example, in U.S. Pat. No. 3,801,326.
In the second process and apparatus described above an extremely long pipe line is required in order to give large grains, since the residence time of the emulsion in one part is short. When the flow rate of the emulsion is lowered so as to prolong the residence time, the mixing of the emulsion with the adjacent solution is accelerated by stirring, causes a wide grain size distribution. In a stirring means and an addition system in a closed system, furthermore, it is required to prevent solution leakage from the joint portion of the apparatus, which is undesirable. Furthermore, this apparatus differs from the small scale apparatus used for experimental research which makes production scale up difficult; the constant pipe length causes poor adaptability to various formulations differing in formulation period; and only limited stirring and mixing performance is achieved.
Further there is the most basic problem, that an excellent process for producing an AgX emulsion established with the use of a small scale reaction apparatus used for experimental research (hereinafter referred to as small scale apparatus) should be applicable to the mass-production of the emulsion (usually performed by using a reaction vessel more than 600 liters in capacity) for commercial purposes.
Such production scale-up is particularly difficult for the following reasons:
(1) When an AgX emulsion is produced according to a specific production procedure in the small scale apparatus, the characteristics of the AgX emulsion differ from those of an AgX emulsion produced in a large scale reaction apparatus (hereinafter referred to as large scale apparatus). In the case of mass-production, therefore, some portion of the production procedure is frequently modified so as to match the properties of these emulsions with each other, which requires a great cost and a long time. This problem is particularly serious in the production of tabular emulsion grains having parallel twin planes.
(2) When a large amount of an emulsion, which is to be on sale in small portions, is produced at one time, some portion of the emulsion obtained should be discarded. This is because the use period of light-sensitive materials is limited and thus no stock is permitted. Accordingly, it is highly desirable to use a process whereby any emulsion can be produced in the needed amount, according to the demand in the market.
(3) In a batchwise process for the mass-production of an AgX emulsion for photosensitive materials, procedures from nucleation to crystal growth are usually performed in a single large vessel for a long period of time and thus a large amount of the emulsion is obtained at once. When it is impossible to use a large amount of the emulsion at once, however, most of the emulsion is divided into small portions and stored in a refrigerator, which requires additional efforts as well as a refrigerating cost. Further, it is required to warm the emulsion prior to the coating, which makes the process further complicated. Accordingly, it is highly desirable to develop a process for producing an AgX emulsion in a needed amount at short intervals of time, rather than producing a large amount of the emulsion at long intervals of time.